Remote actuation of garage door operators and similar loads has been accomplished traditionally by means of a radio control system wherein transmitters and receivers are matched to one another by frequency selection. An inherent disadvantage of this approach is the limited number of available carrier frequencies and the possibility of a match between transmitter and operator belonging to different persons.
One approach to a pulse code system which provides greater latitude in the number of available codes is disclosed in the patent to Willmott U.S. Pat. No. 3,906,348, issued Sept. 16, 1975. Willmott teaches the use of a transmitter having an eleven-stage shift register which is loaded in parallel with a selected pulse code by a plurality of two-position switches. The code is converted to serial by clocking the contents of the register onto a single line which controls an RF oscillator. The output is a series of width modulated pulses wherein bits of one value are represented by pulses of one width and bits of another value are represented by pulses of another width. The receiver/decoder of Willmott comprises a ten-bit shift register which receives the pulse series, another set of two-position switches which are programmed to define the admissible code and a network of gates to permit an instantaneous parallel comparison between all bits in both the received and preselected codes to determine whether of not an admissible code has been received. If all bits compare for three or more consecutive transmission cycles, the load is directly actuated by the output of the comparator.